The invention relates to ammeters of the type employing a magnetic core that is hinged or otherwise mechanically adapted to be located around a current carrying conductor. The conductor current induces a magnetic flux in the core whose magnitude varies with that of the current. A flux sensing device such as a moving coil galvanometer respond to the magnitude of the magnetic flux to produce a signal indicative of the magnitude of the conductor current. Such ammeters are well known.
The measurement made by such an ammeter is significantly affected by the "magnetic history" of the core. Once a magnetizing force has been applied to the magnetic core of the ammeter, the magnetic characteristics of the core, namely, the residual magnetic flux and the relationship between core flux and magnetizing force, are changed. The residual magnetic flux tends to produce an offset in the signal when no external magnetizing force is applied to the core and produces an offset in the signal generated by the flux sensor. The resulting relationship between core magnetic flux and magnetizing force will affect the scaling of the signal generated by the flux sensor. Because both the residual magentic flux and relationship between core magnetic flux and magnetizing force will depend on previous current measurements and in particular on the most recent maximum flux of a particular polarity produced in the core, consistent measurements cannot be taken with such an ammeter.
The effect of the magnetic history of the core of a clamp-on ammeter on the accuracy of measurements is recognized in U.S. Pat. No. 4,059,798 which issued to Diercker et al on Nov. 22, 1977. In the Diercker patent it was proposed that a coil be wound about the magnetic core and periodically energized with a damped oscillatory current wave form to restore the magnetic core to a substantially unmagnetized state.
An alternative approach was taken in U.S. Pat. No. 3,706,032 which issued on Dec. 12, 1972 to Vikstrom. In the Vikstrom patent it was proposed that the magnetic core of a DC clamp-on ammeter be made restorable to a predetermined magnetic condition by winding a resetting coil about the core and providing means for energizing the resetting coil to generate a predetermined magnetic field in the core to produce a residual flux that ensures operation substantially along the outermost hysteresis loop of the core. This would ensure that core magnetic flux would vary in a reproducible manner with the magnetizing force generated by a conductor current. It was proposed that the magnetic core be made momentarily separable to vary the reluctance of the core in a predetermined fashion. A sensor responsive to any resulting change in core flux could then produce a signal indicative of conductor current. Because the net change in core flux is to be measured, the residual flux in the magnetic core would tend not to affect measurements. However, since mechanical means are required to change the reluctance of the magnetic core, it may be expected that the accuracy of measurements will vary with wear or aging of the mechanical separating means.